1. Field of the invention
The present invention relates to a liquid crystal display and, more particularly, to a liquid crystal display which exhibits an increased contrast ratio while maintaining a wide viewing angle.
2. Description of Related Art
Liquid crystal displays are used in various fields, from home use to medical use, because of their features such as low profile, light weight, and low power consumption; furthermore, their use is on the rise. Generally, in a liquid crystal display, a polarizing plate, liquid crystals, and a further polarizing plate are arranged in this order on a backlight. The liquid crystal display controls the quantity of light passing through the polarizing plate on the exit side by applying a voltage to the liquid crystal and performs display of an image.
The viewing angles of liquid crystal displays are steadily increasing due to previous technical developments, including in-plane switching mode (IPS mode) and vertical alignment mode (VA mode) in medical monitors, flat-screen televisions and the like. In-plane switching mode liquid crystal displays are preferred at present because of the smoothness of their viewing angle characteristics.
However, the demands for higher image quality on medical liquid crystal monitors, flat-screen liquid crystal televisions and the like continue to grow. Examples of requirements for higher-quality display include an increase in contrast ratio (the ratio between a luminance with which black is displayed (black luminance) and a luminance with which white is displayed (white luminance)). The contrast ratio increases with increasing white luminance and decreasing black luminance. That is, as the contrast ratio increases, the difference between white and black becomes clearer, and image quality improves. In particular, a contrast ratio of 1,000 or higher is strongly desired for medical monochrome liquid crystal monitors.
A technique for increasing the contrast ratio in a liquid crystal display is described in JP11-337922 (reference 1).
In the liquid crystal display of reference 1, only those light components whose angles of incidence are less than a predetermined angle relative to the perpendicular are permitted to impinge on the pixels from the backlight. More specifically, by limiting the angle of light incident on the liquid crystal panel, the light incident in a more oblique direction is reduced, and light scattering that would otherwise occur within the liquid crystal panel is prevented. As a result the black luminance is reduced, and in turn the contrast ratio is increased.
However, since the liquid crystal display disclosed in reference 1 limits the angle of incident light in all directions parallel to the substrate, the increase in contrast ratio comes at the expense of reduced viewing angle and reduced luminance. The decreases in viewing angle and luminance are serious drawbacks, especially in liquid crystal panels intended to have a wide viewing angle such as the in-plane switching mode displays.